1. Field of the Invention
The present invention relates to an optical branch device for branching signal light in optical communication or the like.
2. Description of the Related Art
In optical communication or like, an optical branch device for branching input signal light into plural components is used as the need arises.
FIG. 23 is a plan view of waveguides of a conventional 4-branched device. In this conventional 4-branched device, a basic waveguide 1 is branched into 4 waveguides 2a, 2b, 3 and 4 at a time. FIG. 24 is a graph showing dimensions of the waveguides when optical outputs of the branched waveguides are simulated about the conventional 4-branched device illustrated in FIG. 23. The transverse axis therein represents the length X in the transverse direction, and the vertical axis represents the length Z in the vertical direction.
FIG. 25 is a graph showing results of the simulation. The transverse axis therein represents the waveguide width of each of the waveguides, and the vertical axis therein represents the output ratio of the inside waveguides 2a and 2b to the outside waveguides 3 and 4 (the ratio of the inside output/the outside output). Δn's each show the refractive index difference between the core of the waveguides and the clad thereof. As shown in FIG. 25, the ratio of the inside output/the outside output varies with a change in the waveguide width. The ratio of the inside output/the outside output varies with a change in the Δn. In optical branch devices, it is desired that optical outputs from their respective branched waveguides are equal to each other and uniform. However, as shown in FIG. 25, in conventional optical branch devices, outputs from their respective branched waveguides are largely varied in accordance with fluctuation in the waveguide width thereof or the refractive index difference (Δn) between their core and their clad. Accordingly, outputs from the respective branched optical waveguides are varied by variation in production conditions. Thus, there is a problem that devices having a uniform quality cannot be produced with a high yield.
As a conventional 4-branched device, known is a 4-branched device wherein one waveguide is first branched into three waveguides and subsequently the central waveguide is further branched into two waveguides (JP-A-8-271744 and so on). In such an optical branch device, it is preferred that one waveguide is firstly branched into three waveguides so as to make the optical intensity of the central waveguide two times larger that of the waveguides on both sides thereof, that is, so as to set the ratio between outputs therefrom to 1/2/1, and subsequently the central waveguide is branched into two waveguides so as to set the ratio between optical intensities therefrom to 1/1, whereby the finally-branched four waveguides emit optical signals at an optical output ratio of 1/1/1/1.
FIG. 26 is a plan view of the initially-branched, 3-branched portion in the 4-branched device. Three branched optical paths 2, 3 and 4 are branched in the form of straight lines from a basic waveguide 1. About this 3-branched waveguide, it is known that when the branch angles θ between the waveguide 2 and the waveguide 3 and between the waveguide 2 and the waveguide 4 are set to 1° or less, the optical intensity ratio of the branched path 3/the branched path 2/the branched path 4 becomes a ratio of 1/2/1 (Correspondence course lecture text “Introductory Lecture, Basis and Actual Application of Optical Waveguide Design”, published by Technical Information Institute Co., Ltd., Second Section, pp. 4–5).
Accordingly, in a 3-branched waveguide as illustrated in FIG. 26, the branch angles θ therein are set into 1° or less and then the central branched waveguide is further branched into two, whereby a 4-branched device wherein outputs from the respective branched waveguides are equal to each other can be yielded. FIG. 27 is a plan view of such a 4-branched waveguide device. In the 4-branched waveguide device illustrated in FIG. 27, the branch angles θ are set to 1°, and further the interval between any adjacent two out of the 4 branched paths is set to 250 μm in order to connect the paths to an optical fiber array. Optical fiber arrays are widely used in the field of optical communication, and are devices wherein optical fibers are arranged at a pitch of 250 μm. For this reason, the length of the branched paths becomes 21.5 mm. Thus, a very long 4-branched waveguide device is unfavorably formed. If the length of waveguides becomes long in this manner, a large insertion loss (−10 log (the ratio of the output/the input) (unit: dB)) is unfavorably generated.